a) Field of the Invention
The present invention relates to a zoom photographic optical system which is to be used in an optical apparatus equipped with an objective lens system for microscope and capable of changing a size of a specimen image to be formed on an image pickup device.
b) Description of the Prior Art
A conventional optical apparatus such as a microscope permits carrying out observation and photographing in procedures described below. First, a specimen is searched for a location to be observed in a broad visual field with an objective lens system having a low magnification. Then, the objective lens system is exchanged (replaced) with an objective lens system having a high magnification and an image of an object (the specimen) is magnified. The magnified image of the object is observed as it is or photographed using an image pickup apparatus. Magnification levels range from 1xc3x97 or so to 100xc3x97. Various methods have been proposed for observing and photographing objects in such a wide magnification range.
As conventional examples of photographic lens system which projects a specimen image formed by a microscope to a silver salt film or the like, there are known photographic lens systems for microscope which are disclosed by Japanese Patent Kokai Publication No. Hei 5-119265, No. Hei 6-281865 and No. Hei 10-62692. The photographic lens systems for microscope disclosed by these patents have magnifications of 2xc3x97 to 5xc3x97.
Furthermore, known as photographic lens systems having low magnifications and vari-focal lens systems for TV photographing are vari-focal lens systems which are capable of photographing at magnifications from 0.25xc3x97 to 4xc3x97 or so in combination with imaging lens systems.
Furthermore, known as conventional examples of imaging lens system which is disposed after an objective lens system for microscope and configured as a zoom lens system are lens systems disclosed by Japanese Patents Kokai Publication No. Hei 4-304409, No. Hei 9-274137 and No. Hei 7-56087. Furthermore, known as conventional examples of optical system which splits a light bundle emerging from an objective lens system and uses photographic optical systems having different magnifications and disposed in split light bundles are optical systems disclosed by Japanese Patents Kokai Publication No. Hei 8-190056 and No. Hei 11-183124.
Furthermore, there is known a method for observing and photographing an image of an object using an a focal zoom lens system for stereoscopic microscope.
Out of the conventional examples mentioned above, the photographic optical systems disclosed by Japanese Patents Kokai Publication No. Hei 5-119265, No. Hei 6-281865 and No. Hei 10-62692 have fixed photographic magnifications. Accordingly, these optical systems require exchanging photographic optical systems which are disposed in photographic optical paths with photographic optical systems having different magnifications when objects are to be photographed at different magnifications, thereby being low in operability.
Furthermore, out of the conventional examples mentioned above, the optical systems disclosed by Japanese Patent Kokai Publication No. Hei 4-304409 and No. Hei 9-274137 use imaging lens systems which are to be disposed after objective lens systems and configured as zoom lens system. These conventional optical systems have a merit to permit continuously changing photographic magnifications with the imaging lens systems (zoom lens systems). However, these optical system do not permit observations and photographing at low magnifications since the optical systems are configured as vari-focal systems for enhancing magnifications. Furthermore, the optical systems have low vari-focal ratios of 2 to 3.
Furthermore, the optical system disclosed by Japanese Patent Kokai Publication No. Hei 7-84189 is a zoom lens system having a vari-focal ratio on the order of 10. However, this zoom lens system is not so preferable since the zoom lens system allows a location of an exit pupil to be varied. Furthermore, the zoom lens system has a large numerical aperture on an exit side at a low magnification since a pupil of an objective lens system is not located in the zoom lens system. As a result, a marginal portion is remarkably eclipsed on an image surface. Furthermore, the zoom lens system has a numerical aperture on the order of only 0.6 at a high magnification and is not so sufficient in resolution. Furthermore, the zoom lens system has a long back focal length, thereby enlarging an optical apparatus.
Furthermore, each of the conventional examples disclosed by Japanese Patents Kokai Publication No. Hei 8-190056 and No. Hei 11-183124 are configured to split an optical path into an optical path on a low magnification side and an optical path on a high magnification side. Accordingly, these conventional example have a merit to facilitate switching between a low magnification and a high magnification. However, these conventional example are configured for observations at fixed magnifications and low in operability.
An object of the present invention is to provide a zoom photographic optical system which has a broad vari-focal range from a magnification lower than a magnification of an objective lens system to a magnification exceeding 10xc3x97. Another object of the present invention is to provide a zoom optical system which is capable of limiting a size of a light bundle at a low magnification. Still another object of the present invention is to provide a zoom photographic optical system which suppresses a variation of an exit pupil.
A zoom photographic optical system according to the present invention is an optical system which is to be used in an optical apparatus equipped with an objective lens system for forming a final image of a specimen, comprises, in order from a side of the objective lens system, a first lens unit having positive refractive power, a second lens unit having negative refractive power and a third lens unit having positive refractive power, changes a magnification from a low magnification position to a high magnification position by moving the above described second lens unit and third lens unit along an optical axis so that an airspace between the first lens unit and the second lens unit at the high magnification position is narrower than that at the low magnification position and an airspace between the second lens unit and the third lens unit at the high magnification position is longer than that at the low magnification position, forms an intermediate image between the first lens unit and the second lens unit at the low magnification position, and satisfies the following conditions (1) and (2).
1xe2x89xa6FH/FLxe2x89xa63xe2x80x83xe2x80x83(1)
3xe2x89xa6MGH/MGLxe2x89xa620xe2x80x83xe2x80x83(2)
wherein a reference symbol FH represents a focal length of lens units as a whole as counted from the first lens unit to another lens unit located just before the intermediate image at the high magnification position, a reference symbol FL designates a focal length of lens units as a whole as counted from the first lens unit to another lens unit located just before the intermediate image at the low magnification position, a reference symbol MGH denotes a magnification of lens units as a whole which are located between the intermediate image and a final image at the high magnification position, and a reference symbol MGL represents a magnification of lens units as a whole which are located between the intermediate image and the final image at the low magnification position.
Moreover, the zoom photographic optical system according to the present invention further satisfies the following conditions (3), (4) and (5):
0.3 less than D1/D0 less than 0.7xe2x80x83xe2x80x83(3)
0.15 less than D2/D0 less than 0.7xe2x80x83xe2x80x83(4)
0 less than FB/D0 less than 0.3xe2x80x83xe2x80x83(5)
wherein a reference symbol D0 represents a distance as measured from a surface of the first lens unit which is nearest the objective lens system to the final image, a reference symbol D1 designates a distance as measured from the surface of the first lens unit which is the nearest the objective lens system to the intermediate image at the low magnification position, a reference symbol D2 denotes a distance for which the second lens unit is moved to change the magnification from the low magnification position to the high magnification position, and a reference symbol FB represents a distance as measured from a surface of a lens element which is nearest the final image to the final image.
Furthermore, the zoom photographic optical system according to the present invention is configured to form an image conjugate with a pupil of the objective lens system at a location between the above described intermediate image and final image, locate an aperture stop at a location of the above described conjugate image, and vary a diameter of the above described aperture stop,